Thin film deposition apparatus and techniques are used, among other techniques, to provide thin film layers on semiconductor substrates. An environment in which desired materials are transported in an ultrahigh vacuum to condense on a clean substrate is a preferred processing technique in the fabrication of microdevices. One well known prior art deposition process is physical vapor deposition, generally referred to as sputtering, wherein substrates are typically moved through load locks and into high vacuum processing chambers which enclose the substrate and a target composed of the material desired to be deposited on the substrate. A negative voltage applied to the target excites into a plasma state an inert gas (typically argon) supplied to the chamber, and ions from the plasma bombard the target and eject particles of target material from the target toward the substrate. These particles deposit on the substrate to form the desired film. In some sputtering process applications, the substrate may be heated to temperatures on the order of about 350.degree. C. to about 510.degree. C. or higher, to reflow a film layer deposited on the substrate by sputtering.
The low pressure, high temperature environments typically utilized in deposition processes cause outgassing of contaminants from the substrates and from internal walls of the processing chamber. These contaminants, such as hydrogen (H.sub.2), water (H.sub.2 O) and air (mostly O.sub.2 and N.sub.2), are released slowly and are often detrimental to the film layer which is deposited onto the substrate. Processes for removing contaminants from substrates prior to processing have been developed and typically include degas chambers located near the front of a cluster tool or otherwise in the preliminary steps of substrate processing.
Contaminants which are outgassed from the processing chamber walls should also be removed from the processing chambers to improve the processing environment. However, outgassing of the chamber walls continues for many hours and it is not feasible to draw out all of the contaminants prior to processing of wafers in the processing chamber. A typical method for reducing outgassing from a vacuum chamber is a baking procedure, during which the chamber is heated to a temperature typically in the range of 120.degree. C.-300.degree. C. for several hours. The thermal energy helps to desorb gas species from chamber walls and the desorbed species are pumped away. A much lower chamber base pressure is thus achieved. The chamber pressure is raised to processing pressure by adding a clean gas during processing. Since contamination due to outgassing is very small, a clean processing environment is obtained.
Processing systems for work pieces such as semiconductor substrates incorporate multiple, isolated vacuum stages between the cassette load lock station and the main vacuum processing chambers. A multistaged vacuum is used between the cassette load lock and the main processing chambers to minimize cross-contamination and allow the use of a ultrahigh vacuum (.ltoreq.10.sup.-8 Torr) in the processing chambers without lengthy pumpdown times. Pre-treatment chambers, such as degas chambers, are typically incorporated in the transport paths of substrates. One such processing system is the Endura.RTM. Platform which is commercially available from Applied Materials, Inc. The Endura.RTM. Platform includes a staged vacuum architecture for achieving very low pressures. Pre-treatment chambers can be dedicated to pre-treating (e.g., plasma etch cleaning, vacuum cleaning and/or heating) of the substrates before processing. A combination of a roughing pump and a cryogenic pump is typically used to provide the vacuum in the degas chambers and the processing chambers. The roughing pump can reduce the chamber pressure to about 10.sup.-3 torr, and the cryogenic pump can then reduce the chamber pressure to about 10.sup.-8 torr.
Sputter deposition of aluminum films on semiconductor substrates is an application in which an ultra-high vacuum at high temperatures is especially desirable. The ultra high vacuum is used to achieve low outgassing of contaminants and removal thereof from the chamber. The chamber pressure for sputtering, commonly as high as the milli-torr range, is achieved by first evacuating the chamber to an ultra high vacuum pressure as low as the 10.sup.-8 torr range to reduce outgassing of contaminants from the chamber walls, and then re-filling ("back filling") the chamber with a clean process gas at a total chamber pressure regulated at about 10.sup.-3 torr.
The time required to pump down processing chambers to the 10.sup.-8 torr range occupies a significant portion of the process time for each substrate, and the inability to achieve lower pressures hinders the removal of contaminants. Therefore, there is a need for a method and apparatus for reducing outgassing of contaminants from the processing chamber walls. By reducing the outgassing of contaminants, the quality of the deposition layers, and devices formed as a result of such deposition, will be greatly improved.